Sulfate assimilation is the process by which environmental sulfur is fixed into organic sulfur for use in cellular metabolism. The two major end products of this process are the essential amino acids cysteine and methionine. These amino acids are limiting in food and feed; they cannot be synthesized by animals and thus must be acquired from plant sources. Increasing the level of these amino acids in feed products is thus of major economic value. Key to that process is increasing the level of organic sulfur available for cysteine and methionine biosynthesis.
Multiple enzymes are involved in sulfur assimilation. These include high affinity sulfate transporter and low affinity sulfate transporter proteins which serve to transport sulfur from the outside environment across the cell membrane into the cell (Smith et al. (1995) PNAS 92(20):9373–9377). Once sulfur is in the cell, sulfate adenylyltransferase (ATP sulfurylase) (Bolchia et al. (1999) Plant Mol. Biol. 39(3):527–537) catalyzes the first step in assimilation, converting the inorganic sulfur into an organic form, adenosine-5′ phosphosulfate (APS). Next, several enzymes further modify organic sulfur for use in the biosynthesis of cysteine and methionine. For example, adenylylsulfate kinase (APS kinase) catalyzes the conversion of APS to the biosynthetic intermediate PAPS (3′-phosphoadenosine-5′ phosphosulfate) (Arz et al. (1994) Biochim. Biophy. Acta 1218(3):447–452). APS reductase (5′ adenylyl phosphosulphate reductase) is utilized in an alternative pathway, resulting in an inorganic but cellularly bound (bound to a carrier) form of sulfur (sulfite) (Setya et al. (1996) PNAS 93(23):13383–13388). Sulfite reductase further reduces the sulfite, still attached to the carrier, to sulfide and serine O-acetyltransferase converts serine to O-acetylserine, which will serve as the backbone to which the sulfide will be transferred to from the carrier to form cysteine (Yonelcura-Sakakibara et al. (1998) J. Biol. Chem. 124(3):615–621 and Saito et al. (1995) J. Biol. Chem. 270(27):16321–16326).
As described, each of these enzymes is involved in sulfate assimilation and the pathway leading to cysteine biosynthesis, which in turn serves as an organic sulfur donor for multiple other pathways in the cell, including methionine biosynthesis. Together or singly these enzymes and the genes that encode them have utility in overcoming the sulfur limitations known to exist in crop plants. It may be possible to modulate the level of sulfur containing compounds in the cell, including the nutritionally critical amino acids cysteine and methionine. Specifically, their overexpression using tissue specific promoters will remove the enzyme in question as a possible limiting step, thus increasing the potential flux through the pathway to the essential amino acids. This will allow the engineering of plant tissues with increased levels of these amino acids, which now often must be added a supplements to animal feed.